Description of the Prior Art
This invention, in its preferred form, relates to apparatus and a related method useful in connection with the production of fuel rod grids, particularly for causing vanes which extend at an angle from some of the straps forming the grid to be deflected to enable them to enter holes in a welding plate and to align segments of the straps.
Nuclear fuel bundle assemblies include a matrix of nuclear fuel rods which are arrayed in rows and columns, and which are held in the desired configuration by a plurality of fuel rod grids. These grids are produced from "straps" which are linearly extending, generally rectangular elements, characterized by having slots extending from one edge approximately half way through the depth of the strap. The straps are assembled so that one strap is in mating relationship with the other strap. Thus, the slot of one strap engages the other strap at a portion thereof which is in alignment with the slot of that other strap, with the result that the grid is of the same depth as each of the straps which forms the grid. The resulting grid has a first set of straps which are substantially parallel to each other, and equally spaced, and a second set of straps which are substantially parallel to each other and equally spaced, the straps of one set being perpendicular to the straps of the other set. All of the aforesaid straps are designated as "inner straps", and they are placed in mating relationship to form a square grid of square cells, in the above noted rows and columns. In addition, there are provided outer straps, which are placed on the four sides of the grid.
The inner and outer straps, made of Inconel, were provided with vanes, which extended from them at a substantial angle, for the purpose of causing water passing through the cells to be given a swirling, turbulent motion. Once the inner and outer straps were assembled to form a grid, brazing material was supplied at juncture points, and the assembled grid, with the brazing material, was positioned in an oven, where brazing was effected and thereby the joining together of the straps to form a sturdy grid.
The above-described method was satisfactory, but it involved the utilization of straps of Inconel. It has been discovered that Inconel results in less efficient use of the nuclear fuel than is desirable, and so there has been developed fuel rod grid straps made of Zircaloy. This material has the advantage that it permits a more efficient use of the nuclear fuel. However, Zircaloy cannot be joined by brazing, since it cannot be exposed to the temperatures necessary for brazing. As a result, it was found necessary to go to a technique of welding the Zircaloy straps in which a laser beam is utilized. The use of a laser beam requires that the straps be held relatively precisely, that is, that the straps of each set be substantially parallel, with their edges straight, the straps of the other set being also so held, and the straps of the two sets being perpendicular. These requirements led to the development of so called "welding plates", one of which was positioned against the side of the grid having the vanes thereon.
Although theoretically each of the straps is planar, in fact, the straps are dislocated from the theoretical planar configuration. It is to be recalled that the straps are made of Zircaloy, and are stamped from a long strip of Zircaloy material. This stamping mode together with subsequent bending operations produces stresses in the material. These stresses, in some instances, have a tendency to cause the straps not to occupy a true planar configuration. In addition, the straps must be handled during processing, thereby encountering various disturbing forces which contribute to the non-planar configuration of them.
The non-planar configuration has been found to be most noticeable in connection with the mating of the grid with a welding plate, called the "vane side welding plate". The vane side welding plate has a plurality of access holes extending through it to permit a laser beam to pass through the holes, to weld the points of intersection of the grid. Because the laser beam and the holding and positioning apparatus for the assembled welding plate and grid must position the grid precisely, to permit welding by the laser beam, the locations of the points of intersection of the straps forming the grid must be precise. The welding plates are therefore provided for this purpose, and include the above-noted laser beam access holes, and two sets of parallel grooves, for receiving the edges of the straps. The straps, along one edge, are provided with vanes, and one set of the straps has slots extending from that edge approximately half way through the depth of the strap, to accommodate a strap of the other set of straps. The slots thereby divide the straps of the one set into segments, and the above-noted stresses, from stamping, handling, and annealing, etc., cause the segments to be non-coplanar. Thus, the straps of the noted set will possibly have the segments thereof out of alignment, at least in part. Further, the holes for the laser beams in the vane side welding plate are as large as possible, but the size of the holes is limited because the strength of the welding plate is diminished by the holes, and beyond a certain size of holes, the welding plate would have a strength which would be too low. Consequently, should the vanes, which extend at an angle to the plane of the strap, be at too great an angle, or otherwise not aligned with the holes, the result will be that the vane side welding plate will strike one or more of the vanes, and bend them, thus preventing the mating relationship which is required between the grid and the vane side welding plate.